1. Field of the Invention
The invention relates generally to a method of modifying the tin to lead ratio in a tin-lead bump already formed on a wafer.
2. Description of the Related Art
At the manufacturing stage where an integrated circuit (IC) chip is to be packaged, the commonly called “first level package” usually refers to the mount and connection of the IC chip on a carrier. This mount is generally performed according to three configurations: wire bonding, tape automatic bonding (TAB), or flip chip (F/C) connection. TAB and F/C connection conventionally require the formation of conductive bumps on the bonding pads of the IC chip for mounting and connecting the IC chip on the carrier. Various types of conductive bumps are known in the art, including a gold bump, a conductive polymer bump, a polymer bump, etc. The person skilled in the art however knows that conductive bumps made of tin-lead alloy are the most commonly met in the currently manufactured IC packages.
To form a tin-lead bump, an under bump metallurgy (UBM) is first formed on the bounding pad of the semiconductor wafer via evaporation, sputtering, or plating. A photoresist layer then is formed over the semiconductor wafer, the photoresist layer having an opening that exposes the UBM. A solder material is filled in the opening of the photoresist layer over the UBM. The solder material is reflowed to form the tin-lead bump.
The formed bump has a tin to lead ratio that determines various physical properties of the bump, including its reflow point. This tin to lead ratio is usually obtained at the preparation stage of the solder material before it is filled in the opening of the photoresist layer.
The U.S. Pat. No. 5,803,340, which is incorporated by reference herein, discloses a composite solder paste for flip chip bumping that is formed via mixing a first solder material of lower reflow point and a second solder material of higher reflow point. This composite solder paste is filled in the opening of the photoresist layer and is reflowed at the lower reflow point to form an initial bump. The photoresist layer then is removed, and the semiconductor wafer is diced into individual chips. An individual chip then is mounted on a substrate with the initial bump mounted on a contact pad of the substrate. The initial bump then is reflowed at the higher reflow point to form the final bump that electrically connects the chip to the substrate.